1. Field of the Invention
The present invention relates to an antenna device comprising a glass antenna formed on a window glass panel of a motor vehicle, particularly to an antenna device comprising a glass antenna formed on a rear window glass panel on which a defogger is provided.
2. Related Art
In Japanese Patent Publication No. P2003-500870A, there is disclosed a glass antenna device for a motor vehicle comprising an AM antenna and an FM antenna on a rear window glass panel provided with a defogger for defogging. FIG. 1 shows the glass antenna device disclosed in the above-described publication.
In this glass antenna device, an AM antenna 12, an FM antenna 14, and a defogger 16 are formed on a rear window glass panel 10.
The AM antenna 12 is composed of an antenna pattern consisting of a plurality of linear conductors arranged in a horizontal direction like a fork shape. The sensitivity of the AM antenna 12 is determined by the area of an antenna pattern thereof, so that the AM antenna 12 is provided in such a manner that the antenna pattern occupies the most part of a space above the defogger 16 on the rear window glass panel 10.
The FM antenna 14 is formed by one linear conductor extending in a horizontal direction on a space between the antenna 12 and the defogger 16.
The defogger 16 comprises bus-bars 16a and 16b arranged oppositely in an up and down direction on both sides of the rear window glass panel 10, a plurality of heating lines 16c arranged in a horizontal direction between the bus-bars 16a and 16b, and a short-circuit line 16d to connect the plurality of heating lines. The bus-bars 16a and 16b are connected to a direct-current (DC) power supply 20 through choke coils 18a and 18b, respectively.
A feeding terminal 12a of the AM antenna 12 is connected through a lead wire 12b to a two-input amplifier 22, and a feeding terminal 14a of the FM antenna 14 is connected through a leas wire 14b to the amplifier 22. The amplifier 22 is connected to a radio set (not shown) through a coaxial cable 24.
When the AM and FM antennas 12 and 14 are provided in a space other than the defogger on the rear window glass panel 10 and the two-input amplifier 22 is used, the directivity of the FM antenna is remarkably degraded in a given angle. This is due to the fact that while the sensitivity of the FM antenna is tuned by the length of the FM antenna element including the lead wire, a capacitive coupling to heating lines of the defogger, and a capacitive coupling to the AM antenna, the directivity of the FM antenna is affected by the approaching of the AM antenna to a motor vehicle body.